Editor's note: Pete Cashmore is the founder and CEO of Mashable, a popular blog about social media. He writes a weekly column about social networking and tech for CNN.com.

(CNN) -- In the spring of this year, the "Facebook alternative" Diaspora achieved extensive media coverage -- including an article in the New York Times -- and raised tens of thousands of dollars in funding from online donors.

The pitch was an appealing one: In the midst of a privacy backlash at Facebook, Diaspora proposed a more private alternative to the leading social network.

But Diaspora is no Facebook rival, and history tells us it won't make a dent in Facebook's success.

A focus on privacy

Diaspora, which began sending out invites this week, attempts to outclass Facebook in privacy features and user freedom. Features include granular control over who sees your information, the ability to download your photos and the option to delete your account without any hassle.

With $200,000 now raised for the project, you'd think Diaspora was off to a great start. In reality, its impact on Facebook will be minimal.

There are plenty of ways for a social network to fail; the fact that your friends are already on Facebook and not Diaspora is the most obvious issue. But when it comes to toppling web giants, one factor is frequently overlooked: The replacement is usually radically different from its predecessor.

In other words: If Facebook is ever beaten, it won't be by a site that tries to be "Facebook, but better."

Rise of the Digg clones

A cautionary tale emerged this year in the form of Digg.com. The once-pioneering social news site allows users to vote on stories, pushing those articles to the front page. This concept inspired hundreds of "Digg clones," few of which were able to gain traction. Most notably, a much-hyped relaunch of Netscape.com with Digg-like voting wasn't a success.

This year, Digg entered a decline after a series of stumbles, and some users were said to defect to rival Reddit. And yet the site's troubles really began much earlier, with the rise of Twitter and Facebook. Although radically different in their execution, these two social services served much the same purpose as Digg by allowing users to share interesting links they found around the web. As web users found new outlets for these behaviors, Digg's relevance dwindled.

The real threat to Digg wasn't a clone but rather a completely different approach.

MySpace in decline

Likewise, Facebook, which usurped MySpace for the social networking crown, did so with a radical innovation: the launch of its News Feed in 2006. This new feature wasn't just a "better MySpace" but a completely different approach to social interaction that replaced static pages with streams of constantly updated information.

Facebook vs. Google

Perhaps unexpectedly, Google is now feeling the heat from Facebook. Although hundreds of "Google killers" have come and gone, all of them trying to tackle Google head-on, it looks like Facebook might be the most credible threat so far to Google's core search business.

What makes Facebook a possible Google rival is the massive amount of data it possesses on your individual preferences, your network of friends and the links you share. All this information could, the pundits predict, power a search engine with better results than Google.

Social networks pose another threat to Google's search business, too: By subscribing to Twitter feeds and Facebook pages, users have relevant content delivered straight to them, which reduces the need to use search engines. In short: The most likely "Google killers" look nothing like Google.

Diaspora is no threat

So, if recent history is a guide, Facebook need not worry about Diaspora. Such incremental improvements almost never create new market leaders. Instead, it takes a completely different approach to unseat incumbents.

If a true "Facebook killer" should ever emerge, it probably won't resemble Facebook at all.